1. Field of the Invention
The present invention relates generally to the field of cable television (CATV) systems. More specifically, the present invention discloses a system for providing full duplex telephone communications over a CATV network.
2. Statement of the Problem
FIG. 1 shows an simplified overview of a typical CATV architecture as currently implemented. This is commonly referred to as a tree and branch structure. The headend 10, where programming signals originate, is connected by fiber optics 12 to a number of optical network units (ONU's) 15. At the ONU 15, the optical signal is converted to an electrical signal and sent down the distribution network using conventional coaxial cables 16. This cable is split into different paths for distribution to multiple routes with multiple subscriber units 17. If the branches become long enough, line amplifiers (also known as extenders) 18 are inserted at intervals in the coaxial cable network to boost the signal.
If telephone service is desired, the CATV architecture must provide a return path. To implement this return path, each line extender must be equipped with either an amplifier or a bypass strap in the return or upstream direction. Once the return signal reaches the ONU, a diplex filter splits the return signal off and routes it through a return fiber optic back to the headend. Presently, most CATV networks transmit signals in the forward or downstream direction at frequencies from approximately 50 to 600 MHz. The reverse or upstream direction is rarely used. However, when the upstream direction is used for upstream telephony and other interactive services, the frequency range is typically in the range of approximately 5 to 40 MHz.
The frequency range for telephony in the downstream direction is also an issue. Two alternatives have been suggested. The first alternative is to take one 6-MHz TV channel for downstream telephone communications. Many CATV system operators oppose this alternative because they are already shod of space for TV channels. The second alternative proposes to place downstream telephony in a frequency band above 600 MHz. One major disadvantage of this scheme is the fact that almost all existing line extenders would need to be replaced because their maximum bandwidth is approximately 600 Mhz.
Another major disadvantage of both alternatives is the fact that they are dependent on proper operation of the line extenders. The downstream telephony signal as well as the video signal are lost if a line extender fails. Line extender failure is a common occurrence in CATV networks. This question of reliability (as compared to the exceptional reliability that local telephone companies have historically enjoying using a copper-based network) is considered a major concern with consumers based on industry marketing surveys.
A number of systems have been invented in the past to provide telephony or other narrow-band communications over CATV networks, including the following:
______________________________________ Inventor Patent No. Issue Date ______________________________________ Shomo 3,943,447 March 9,1976 Mertel 3,980,831 Sept. 14, 1976 Toriumi 4,454,538 June 12, 1984 Lehman et al. 4,763,317 Aug. 9, 1988 Sahara et al. 4,805,014 Feb. 4, 1989 van Baardwijk et al. 4,811,335 March 7, 1989 Ohue 4,928,272 May 22, 1990 Graves et al. 5,029,333 July 2, 1991 Maki 5,124,980 June 23, 1992 Burton et al. 5,189,673 Feb. 23, 1993 ______________________________________
Shomo discloses a system for bidirectional communication over existing CATV networks. Communications in the frequency band of 10 to 300 KHz follow the path of the 60-Hz power signal throughout the system. This allows bidirectional communications between any two points linked by the CATV network without requiring substantial modifications or additions to existing CATV system components.
Mertel discloses a data switching system in which a number of subscriber sets are connected over a broad-band transmission line to a switching center. The system handles audio and video telephone conversations, television communications, and data communications between the connected parties. Each subscriber set includes a control device connected over the subscriber line that selects a desired mode of operation.
Toriumi discloses a system for two-data communications over a CATV network using frequency division multiplexing. Each terminal unit includes a PLL that oscillates at a frequency corresponding to a channel selected. A frequency divider circuit subjects the output of the PLL circuit to frequency division to provide a carrier frequency for an upstream data signal. The central facility transmits programs and downstream data signals through a plurality of channels. Each terminal unit transmits an upstream data signal to the central facility by using an exclusive frequency corresponding to the channel selected.
Lehman et al. disclose a digital communication network architecture for providing universal information services including wide-band and narrow-band voice, data, and video communications. The proposed system appears to be an implementation of an integrated services digital network (ISDN) that is intended to supersede existing CATV networks.
Sahara et al. disclose an interface between a broadcast satellite system and a CATV network. A receiving stage receives the signal broadcast by the satellite consisting of base-band video signals and subchannel digital audio signals. Demodulators derive the video signals and digital audio signals. A first modulator modulates the main carriers of the CATV channels with the video signals, and a second modulator modulates the main carrier of a different channel with the digital audio signals in a time-division manner. The combined output of the first and second modulators is transmitted through the CATV transmission line.
Van Baardwijk et al. disclose a broadband/narrow-band switching network for use in an ISDN system combining telephony, data, and video communications.
Ohue discloses a two-way CATV system using frequency division multiplexing. A digital transmultiplexer at each junciton between the trunk line and a branch line converts the frequency division multiplex signal into time-division multiplex signals.
Graves et al. disclose a communications system for multiplexing broadband signal components, such as video channels, with multiplexed telephony signals.
Maki discloses a digital two-way communications network using T1 PCM communications over a CATV network.
Burton et al. disclose an integrated telecommunications network adapted to carry telephony signals and broad-band switched video signals. Control signals for the video signals are transmitted in the narrow-band as baseband signals, which are frequency division multiplexed with the broad-band switched video signals. The video control signals are transmitted over the same transmission lines as the broadband video signals and do not require the installation of additional transmission lines. Control signals for subscribers are multiplexed together at a network unit and are transmitted as a channel on an optical fiber, after which the channels from a plurality of network units are further multiplexed together to form a frame of control channels that are transmitted to a central video controller.